The biomechanics of the acetabulum and acetabular replacement
This thesis describes studies performed to measure the deformation of the human acetabulum during weight bearing. This work is then extended to measure the effect of three surface coatings on the relative motion between the pelvis and cementless acetabular cups. The soft-tissue and bony anatomy of the hip joint is described with particular attention to the muscles controlling joint motion. An additional chapter is devoted to the biomechanics of gait and electromyographic measurement of muscular activity during normal walking. This is followed by a review of the biomechanics of the hip joint and the contribution of studies performed using instrumented hip prostheses. The author then presents a new approach to the prediction of muscle forces during walking. A conventional engineering analysis is presented relating the intersegmental components of force and moment acting across the hip joint to forces developed by individual muscles during the early stance phase of the gait cycle. Various strategies are employed to solve this indeterminate mechanical problem using data derived from quantitative electromyography, the kinematics and biomechanics of gait and intravital recordings of hip joint forces. Through use of mathematical optimization, a solution is found that is consistent with both the engineering analysis and quantitative electromyographic data. Experiments are reported utilizing four cadaveric specimens in which the position and loading of the hip joint were recreated using instrumented cables and loading fixtures. Measurements of acetabular deformation are reported at 11 sites on each specimen. These data are recorded on a computer disk which accompanies this thesis. The overall pattern of deformation is found to be consistent with biaxial bending of the pelvis over a fulcrum formed by the femoral head. The second part of the thesis describes an experiment performed to determine whether external coatings influence the stability of cementless acetabular cups implanted in the acetabulum. A standard design of an acetabular cup is developed on the basis of anatomic studies of cadaveric pelves. Experiments are described using cups with three external coatings: spherical Co-Cr beads, plasma-sprayed titanium and 1 hydroxyapatite. These components were implanted into the acetabuli of five fresh cadaveric pelves and loaded in three-point bending. Interface motion was measured at the dome, the acetabular floor and the cotyloid notch of each specimen during loading to 2000 N. The results of this study are recorded on a computer disk (see Appendix). These data showed that the surface coating dramatically influences the interface motion of acetabular cups. Implant motion was found to be site-specific and appeared to be determined, primarily, by the interaction of elastic deformation of the pelvis and frictional conditions at the shell/bone interface.